**3.1 Heat transfer capacity**

**Figure 6** shows the change curve of heat transfer capacity of HGHE with time under different inlet water temperatures. **Figure 6a** shows the change of outlet water temperature of HGHE with running time under four different working conditions. It can be found that outlet water temperature gradually increases with time and finally reaches the stable state. Corresponding **Figure 6b** shows the change of heat flux per unit pipe length with running time under four working conditions. The heat flux per unit pipe length gradually decreases with the passage of time and finally reaches a stable state. Both the outlet temperature curve and the heat flux curve have experienced two stages: the dramatic change stage in the initial stage of operation and the gentle change stage in the later stage. It can be seen that the higher the inlet water temperature is, the more drastic the change at the early stage of operation. This is because the higher the inlet water temperature is, the more heat will accumulate near the spiral pipe in the early stage, which makes the heat exchange resistance of the spiral pipe greater; thus, the heat exchange per unit pipe length decreases more dramatically.

#### **Figure 6.**

*Heat exchange capacity changing with time of different inlet water temperature conditions. (a) Outlet water temperature changing with time. (b) Heat flux per unit pipe length changing with time.*

**Figure 7.** *Heat flux per unit pipe length changing with inlet water temperature.*

In order to more clearly explore the influence of inlet water temperature on the heat transfer capacity of HGHE, the relationship curves between heat flux per unit pipe length and inlet water temperature at different times (24, 72, and 120 h) are made, as shown in **Figure 7**. It can be seen that under the three moments, the heat flux per unit pipe length has a linear increasing trend with the increase of the inlet water temperature. When the heat removal operation reaches 24 h, the heat flux per unit pipe length of HGHE is 5.63, 7.60, 9.56, and 11.52 W/m, respectively, for the inlet water temperature of 32, 35, 38, and 41°C. Under the conditions of four different inlet temperatures (32, 35, 38, and 41°C), the heat flux per unit pipe length of HGHE is 3.83, 5.08, 6.35, and 7.63 W/m, respectively, at the moment of 72 h. When the heat removal operation reaches 120 h, the heat flux per unit pipe length of HGHE is 3.18, 4.22, 5.23, and 6.23 W/m for the inlet water temperature of 32, 35, 38, and 41°C, respectively. It is concluded that when the system runs to the late stable state, the linear increase range of heat flux per unit pipe length of HGHE becomes small with the inlet water temperature. For example, at the moment of 120 h, the heat flux per unit pipe length increases only about 0.34W/m when the inlet temperature increases by 1°C.

#### **3.2 Thermal response of soil**

**Figure 8** shows the temperature distribution field of half longitudinal sections under different inlet water temperature conditions. It can be seen from the figure that the higher the inlet water temperature is, the more obvious the soil heat accumulation phenomenon near the spiral buried pipe is, and the stronger the thermal interference effect is. In particular, the temperature of the soil inside the HGHE increases dramatically with the increase of the inlet water temperature, which is mainly due to the limited volume of soil in the pile. With the progress of the heat removal condition, the heat discharged from the spiral pipe around HGHE will be transferred to the pile, resulting in heat accumulation, while the heat inside HGHE can only be discharged through the upper and lower opening surface. Therefore, the increase of inlet water temperature means that more heat will accumulate inside HGHE, so the thermal interference inside the pile is more and more serious, which will also greatly hinder the improvement of the heat transfer capacity of HGHE. It can also be seen that the

*CFD Applications in Ground Source Heat Pump System DOI: http://dx.doi.org/10.5772/intechopen.109574*

#### **Figure 8.**

*Soil temperature distribution of different inlet water temperature conditions. (a) 32°C, (b) 35°C, (c) 38°C, (d) 41°C.*

increase of inlet water temperature has little influence on the thermal action range of HGHE, but mainly increases the thermal response temperature of soil within the thermal action range. Therefore, it shows that increasing the inlet water temperature of HGHE has no significant effect on improving the heat exchange performance. On the contrary, it will increase the outlet temperature of HGHE, resulting in higher condensing temperature and lower the efficiency of the GSHP system.
